Many image projection systems, such as might be used for projection televisions, are based on the use of liquid crystal display (LCD) imager panels. Some LCD panels operate in a reflective mode, in which incident illumination light is separated from reflected image light by using a polarizing beamsplitter in front of the reflective LCD panel. In such configurations, the illumination light is passed to the LCD panel via the polarization beamsplitter. The illumination light incident at the LCD panel is, therefore, polarized. The LCD panel operates by selectively adjusting the polarization modulation of the many pixels of the panel. Those pixels associated with dark areas of the image do not alter the polarization state of the light whereas those pixels associated with bright areas of the image do alter the polarization state of the light. When the illumination light is reflected to the LCD panel by the polarization beamsplitter as reflected light, that light which has polarization that has been changed to the polarization state that is orthogonal to the polarization state of the incident light is transmitted through the polarization beamsplitter. Ideally, only the light corresponding to pixels that actively modulate the incident light is transmitted through the polarization beamsplitter to the projector's lens system, while light reflected by pixels that correspond to dark areas of the image, i.e. pixels that are not actively modulating the light, is rejected by the polarization beamsplitter. Thus, the beamsplitter can be used to separate the polarization modulated image light from the unmodulated light, which results in an image beam that can be projected.
An important characteristic of a display is the contrast ratio which, qualitatively, is a measure of how bright the bright pixels are compared to the dark pixels. The contrast ratio, RC, can be quantified as:RC=Ib/Id,where Id and Ib correspond to the amount of reflected light in the dark and bright states respectively. Ideally, the value of Id is as small as possible, so that the contrast ratio is large.
However, in certain types of LCD image-forming device, such as twisted nematic (TN), super twisted nematic (STN) and vertically aligned nematic (VAN) panels, the liquid crystal manifests a residual birefringence even in the off (non-modulating) state. This residual birefringence increases the amount of light in the dark state, resulting in a reduction in the image contrast ratio.
While a birefringent retardation plate with the same, but opposite retardation, may be used to compensate for this residual birefringence, a quarter-wave retardation plate, inserted between the polarizing beamsplitter and the LCD panel, can also be used to compensate for the residual birefringence. Furthermore, a quarter-wave retardation plate can also be used to compensate for birefringence in components of the polarizing beamsplitter. A compensating retardation plate is not suitable for compensating birefringence in components of the polarizing beamsplitter, and so the use of a quarter-wave retarding plate is more desirable.
The quarter wave retardation plate is oriented so that the slow or fast axis is rotated away from being parallel to the polarization plane of the illumination light by a few degrees. One disadvantage of this compensation technique, however, is the very careful alignment required to achieve the optimum orientation of the quarter wave retarder. This alignment step increases the costs of producing a projection system.